Most current aircraft possess a flight management system, for example of the FMS type, the acronym standing for the term “Flight Management System”. These systems allow navigational aid, by displaying information useful to pilots, or else by communicating flight parameters to an automatic piloting system.
Notably, a system of FMS type allows a pilot or another qualified person, to input, pre-flight, a flight plan defined by a departure point of the flight plan, an arrival point of the flight plan, and a series of passing points or waypoints, customarily designated by the abbreviation WPT. All these points can be chosen from among points predefined in a navigation database, and which correspond to airports, radionavigation beacons, etc. The points can also be defined manually by their geographical coordinates and optionally their altitude.
The passing points can be input through a dedicated interface, for example a keyboard or a touchscreen, or else by transferring data from an external device.
A flight plan then consists of a succession of segments, or “legs” according to the terminology customarily employed in this technical field.
Other data can be entered into the flight management system, notably data relating to the aircraft's load plan and to the quantity of fuel aboard.
When the aircraft is in flight, the flight management system precisely evaluates the position of the aircraft and the uncertainty in this item of data, by centralizing the data originating from the various positioning devices, such as the satellite-based geo-positioning receiver, the radionavigation devices: for example DME, NDB and VOR, the inertial platform, etc.
A screen allows the pilots to view the current position of the aircraft, as well as the route followed by it, and the closest passing points, the whole on a map background making it possible to simultaneously display other flight parameters and distinctive points. The information viewed allows notably the pilots to adjust flight parameters, such as heading, thrust, altitude, rates of climb or of descent, etc. or else simply to check the proper progress of the flight if the aircraft is piloted in an automatic manner. The computer of the flight management system makes it possible to determine an optimal flight geometry, notably in the sense of a reduction in operating costs related to fuel consumption.
FIG. 1 presents a summary diagram illustrating the structure of a flight management system of FMS type, known from the prior art.
A system of FMS type 100 has a man-machine interface 120 comprising for example a keyboard and a display screen, or else simply a touch-sensitive display screen, as well as at least the following functions, described in the aforementioned ARINC 702 standard:                Navigation (LOCNAV) 101, for performing the optimal location of the aircraft as a function of the geo-location means 130 such as satellite-based or GPS or GALILEO geo-positioning, VHF radionavigation beacons and inertial platforms. This module communicates with the aforementioned geo-location devices;        Flight plan (FPLN) 102, for inputting the geographical elements constituting the skeleton of the route to be followed, such as the points imposed by the departure and arrival procedures, the waypoints, the air corridors (or “airways” as they are commonly known);        Navigation database (NAVDB) 103, for constructing geographical routes and procedures with the help of data included in the bases relating to the points, beacons, interception legs or altitude legs, etc;        Performance database (PRFDB) 104, containing the craft's aerodynamic and engine parameters;        Lateral trajectory (TRAJ) 105, for constructing a continuous trajectory on the basis of the points of the flight plan, complying with the performance of the aircraft and the confinement constraints (RNP);        Predictions (PRED) 106, for constructing an optimized vertical profile on the lateral and vertical trajectory. The functions forming the subject of the present invention affect this part of the computer;        Guidance (GUID) 107, for guiding in the lateral and vertical planes the aircraft on its three-dimensional trajectory, while optimizing its speed. In an aircraft equipped with an automatic piloting device 110, the latter can exchange information with the guidance module 107;        Digital data link (DATALINK) 108 for communicating with the control centres and the various other aircraft 109.        
The flight plan is entered by the pilot, or else by data link, with the help of data contained in the navigation database. A flight plan typically consists of a succession of segments, customarily designated by the name “legs”, which are formed of a termination and of a geometry, for example a geometry of turn type, or else of straight line type as great circle or rhumb line. The various types of legs are defined in the ARINC 424 international standard.
The pilot thereafter inputs the parameters of the aircraft: mass, flight plan, span of cruising levels, as well as a or a plurality of optimization criteria, such as the CI. These inputs allow the modules TRAJ 105 and PRED 106 to compute respectively the lateral trajectory and the vertical profile, that is to say the flight profile in terms of altitude and speed, which for example minimizes the optimization criterion.
The lateral trajectory of the aircraft is thus computed from leg to leg while complying with a certain number of conventions.
For example, for safety reasons, ARINC 424 makes provision, when a change of course (that is to say a change of track) between two consecutive legs of the flight plan is too big (typically greater than 135°), to impose a track for joining the following leg by imposing a turn direction of the aircraft or “Forced Turn Direction” as it is commonly referred to.
Moreover, when stringing the legs together, certain transitions also exhibit a “forced” direction or “Forced Turn Direction”. Thus, the stringing together of leg n and leg n+1 must absolutely be performed by a trajectory complying with an imposed turn direction.
When there is no “Forced Turn Direction” between a leg and the following one, the FMS according to the prior art computes a joining trajectory comprising a turning of the aircraft according to a turn direction dubbed the logical direction or “Logical Turn Direction” (LTD). The LTD is determined in a basic manner, it is related directly and solely to the change of track between the legs.
There exist other situations in which for diverse reasons the aircraft has quit its flight plan and is therefore flying a trajectory outside of the initially computed lateral trajectory. To join the flight plan, the FMS then computes a trajectory for joining an arrival leg on the basis of the current position of the aircraft. In this case also the FMS chooses an initial turn according to the LTD direction.
We shall firstly explain in FIGS. 2a and 2b certain conventions making it possible to define the parameters necessary for a proper understanding of the computation of the LTD according to the prior art.
In FIG. 2a, a departure leg is defined by a departure point Pd and a track according to the direction of movement of the aircraft, corresponding to an oriented straight Dd charted by an angle of departure Ad defined with respect to a reference track Ref, typically North. By convention, the angle is considered positive clockwise, and lies between 0° and 360°. This departure point Pd can be a leg n when computing a lateral trajectory according to a predetermined flight plan, or the current position of the aircraft when computing a joining trajectory of an aircraft that has quitted its flight plan and wishes to join it at the level of a given arrival leg.
For the computation of the trajectory for joining the following leg or arrival leg, the FMS according to the prior art takes into consideration only the straight oriented in the desired direction of arrival of the aircraft on this leg, dubbed the arrival straight Da, without considering the exact position of the arrival point Pa corresponding to the geographical coordinates of the navigation point associated with the leg.
FIG. 2b explains the geometric situation of the departure and arrival legs according to the above conventions.
The system computes a joining trajectory so that an aircraft situated at Pd and flying according to a track corresponding to the angle of departure Ad, joins the straight Da. This trajectory begins with a turning of the aircraft and continues with a typically straight part which intersects the straight Da according to a joining angle AR. Typically the capture of the arrival straight Da is done according to a joining angle of 45° for a civil aeroplane. This angle may reach 90° for a fighter plane capable of performing tight turns. Likewise for flight time optimization reasons, the value of 45° can be decreased in zones of light air traffic.
The direction of a turn is left if the aircraft banks to its left and right if it banks to its right. By convention, a sign is allotted to the direction of the turn: a turn to the left (anti-clockwise) is negative, a turn to the right (clockwise) is positive.
The principle of computing the LTD is illustrated in FIG. 3 for various configurations of departure and arrival courses. The LTD (“Logical Turn direction”) is related directly to the change of track between the legs, that is to say it is dependent only on the angle between Ad and Aa. This angle between Aa and Ad is customarily dubbed the angle of change of course or “Track Change” as it is commonly referred to.
The LTD corresponds to the turn direction which minimizes the amplitude of the change of track from Ad to Aa. The LTD is defined by a sign, positive when it is right (clockwise) and negative when it is left (anti-clockwise). Stated otherwise, the sign of the LTD corresponds to the sign of the difference Aa−Ad, if necessary converted so that this difference is referred back between −180° and +180° (smaller angle in absolute value between Ad and Aa):For 0<Aa−Ad<180°, sign of LTD=+For −180°<Aa−Ad<0°, sign of LTD=−
Thus, according to the prior art the system always chooses joining according to the smallest, in absolute value, “track change” angle, doing so whatever the position Pd of the aircraft with respect to the arrival straight Da.
FIG. 4 illustrates an example in which the direction of the logical turn LTD, computed by the system to join the straight Da, is left, i.e. negative (LTD=−). This logical direction LTD determined by the system is independent of the position of Pd with respect to the straight Da, as illustrated in FIG. 4. The trajectory 40 is the computed lateral trajectory of the aircraft when the departure point Pd is situated on the right of the arrival straight Da and the trajectory 41 is the computed lateral trajectory of the aircraft when the departure point Pd is situated on the left of the arrival straight Da. These two computed lateral trajectories 40 and 41 both begin with a left turn.
Thus the computation of the LTD takes no account of the geometric characteristics of the flight plan, and more particularly of the position of the departure point Pd with respect to the arrival straight Da. This mode of computation thus presents the drawback of not corresponding, for certain geometries, to the natural direction that would be chosen by the pilot or to the direction minimizing the distance travelled by the aircraft to join the straight Da.
Moreover, for certain geometries, an example of which is illustrated in FIG. 5, the joining of the straight Da on the basis of the LTD computed is performed downstream of the arrival point Pa, since the position of the point Pa is not taken into account in the computation of the LTD. Let us consider that the point Pd and the angle Ad correspond to a leg n, and that the arrival point Pa and the angle of arrival Aa correspond to the leg n+1 following the leg n of a flight plan. After leg n+1, the aircraft must join leg n+2 and so on and so forth.
The FMS according to the prior art computes a joining trajectory 50 passing beyond the point Pa and directly joining leg n+2 see n+3 of the flight plan. This computed trajectory is not satisfactory since it is not slaved sufficiently to the flight plan and hinders the pilots as well as the air traffic control.